The present invention relates to friction drive apparatus such as printers, plotters and cutters that feed strip material for producing graphic images and, more particularly, to a method for calibration of friction drive apparatus and a method for automatic alignment of strip material therein.
Friction, grit, or grid drive systems for moving strips or webs of sheet material longitudinally back and forth along a feed path through a plotting, printing, or cutting device are well known in the art. In such drive systems, friction (or grit or grid) wheels are placed on one side of the strip of sheet material (generally vinyl or paper) and pinch rollers, of rubber or other flexible material, are placed on the other side of the strip, with spring pressure urging the pinch rollers and material against the friction wheels. During plotting, printing, or cutting, the strip material is driven back and forth, in the longitudinal or X-direction, by the friction wheels while, at the same time, a pen, printing head, or cutting blade is driven over the strip material in the lateral or Y-direction.
These systems have gained substantial favor due to their ability to accept plain (unperforated) strips of material in differing widths. However, the existing friction drive apparatus experience several problems. One problem that occurs in friction drive apparatus is a skew error. The skew error will arise as a result of strip material being driven unevenly between its two longitudinal edges, causing the strip material to assume a cocked position. The error is integrated in the lateral or Y-direction and produces an increasing lateral position error as the strip material moves along the X-direction. The error is often visible when the start of one object must align with the end of a previously plotted object. In the worst case, such lateral errors result in the strip drifting completely off the friction wheel. The skew error is highly undesirable because the resultant graphic image is usually destroyed.
Most material strips are inserted manually into the friction drive systems. During the manual insertion, it is essentially impossible to place the material strip perfectly straight in the friction drive apparatus. Therefore, the existing systems typically use at least three feet of strip material until the strip material is straightened with respect to the friction drive apparatus. This manual alignment procedure has numerous drawbacks. First, it results in excessive material consumption and waste thereof. Second, the procedure is time consuming. Additionally, manual alignment is not always effective. Therefore, there is a need to reduce wasteful consumption of strip material during loading thereof into the friction drive apparatus and to ensure proper alignment of the strip material within the friction drive apparatus during operation.
It is an object of the present invention to provide an apparatus and a method for automatically aligning strip material in a friction drive apparatus at the onset of an operation without excessive strip material waste.
It is another object of the present invention to provide an apparatus and a method for properly calibrating two sensors that detect an edge of the strip material in the friction drive apparatus with respect to each other.
According to the present invention, a friction drive apparatus includes an edge detection system having a first sensor and a second sensor for determining a lateral position of a longitudinal edge of a strip material. The friction drive apparatus also includes first and second friction wheels advancing the strip material in a longitudinal direction that are rotated by independently driven motors which are driven independently in response to position of the longitudinal edge of the strip material detected by the sensor disposed behind the friction wheels with respect to the direction of motion of the strip material.
The friction drive apparatus also includes instructions for automatically aligning the strip material in the friction drive apparatus upon loading of the strip material and instructions for calibrating the second sensor with respect to the first sensor of the edge detection system. The automatic alignment procedure includes steps of advancing the strip material in the longitudinal direction a predetermined aligning amount while the strip material is steered with respect to the controlling sensor to eliminate any lateral deviations of the strip material from the feed path. The calibration procedure calibrates the second sensor with respect to the first sensor to eliminate any potential offset that may have been introduced during assembly and installation of the sensors.
One advantage of the present invention is that it eliminates the need for an operator to manually align the strip material. The automatic alignment reduces the amount of wasted strip material as compared to a manual alignment operation and results in time savings and improved quality of the final graphic product. Another advantage of the present invention is that the calibration procedure provides additional accuracy to the proper alignment of the strip material and also improves quality of the final graphic product.
The foregoing and other advantages of the present invention become more apparent in light of the following detailed description of the exemplary embodiments thereof, as illustrated in the accompanying drawings.